Who keeps an eye on who?

Is it the Black-headed Gull who keeps an eye on the Tufted Duck or is it the Tufted Duck who keeps an eye on the Black-headed Gull?
I am not so sure it is a Tufted Duck, but I know it is a diving duck, I mean a very good diver.
Someone who can tell me what kind of duck this is? The duck's eyes are yellow.